Mail items are inserted into a postal sorting machine by means of an unstacker which separates the mail items one-by-one from the stack and injects them into a conveyor serving the sorting outlets of the machine. Unstacking is a very important and difficult step in handling mail items. One known type of malfunctioning of a sorting machine is related to the fact that mail items might not be unstacked correctly, and are thus injected as a “multiple take” into the conveyor, i.e. by being mutually superposed or overlapping. This type of malfunctioning can occur during successive sorting passes.
A method is already known from Patent Document DE 19 625 043 for identifying the presence of mail items in multiple takes by combining probabilities on measurements of mail item length, height, and thickness.
Another method is also known from Patent Document GB 2 279 634 for detecting multiple takes by means of a system of belts driven at various speeds combined with measuring mail item length at the inlets and at the outlets of the system of belts.
Other methods of identifying the presence of mail items in multiple takes are also described in Patent Documents U.S. Pat. Nos. 3,955,812, 4,121,716 and FR 2 841 487.